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COATINGSPRO CONCRETE COVERED 2017 31
Concrete and Water
complexity of the mix design. Concrete that is too stiff is
difficult to place uniformly, resulting in voids and weak
layers after placement. Concrete that is too fluid may strat-
ify or "segregate" into layers of coarse aggregate alternating
with layers of high water and fines content. e wrong mix
design (i.e., too much or too little water) is the second latent
construction defect. To facilitate placement and consolida-
tion, vibration is commonly used, but this may incompletely
compact the layers, resulting in " honeycomb" voids in the
concrete, or it may increase segregation and produce non-uni-
form concrete.
Water is also necessary for the curing of concrete. e
hydration of hydraulic cement occurs over a time span of
weeks, with typical design strengths required at 28 or even 90
days after placement. During this hydration period, moisture
is consumed by the cement filling in the spaces between
aggregate particles. If water is not present during this time,
improper hydration of the cement will result in low strength
and poor durability. Improper curing is the third latent
construction defect contributing to concrete issues.
Concrete is strong in compression and comparatively
weak in tension. Reinforcement, usually in the form of steel
bars (aka rebar), is added to improve the concrete's tensile
properties. Steel that is used for concrete contains mainly
iron, which will eventually rust if sufficient water and oxygen
are present. ere is a synergy between the two materials
in steel-reinforced concrete: e alkalinity of the concrete
(high pH) causes a protective (passivating) layer to remain
on the steel/concrete interface that prevents rusting of the
steel. is passivating layer will remain to protect the steel
until the pH around it is reduced, chloride (or other damag-
ing chemicals) enter the concrete and penetrate to the steel,
or stray electrical currents cause the layer to dissipate. Once
the passivating layer is no longer present, rust will form. Rust
occupies a larger volume than the iron, oxygen, and water
that form the rust, creating an expansive force in the concrete
that causes cracking in the covering concrete.
Once cracking occurs, further deterioration is acceler-
ated since now these deleterious materials can easily reach
the steel. erefore, the steel reinforcement must be covered
by enough concrete of suitable quality to provide adequate
protection for the anticipated life cycle of the structure.
Frequently during concrete placement, the reinforcement
may shift or move closer to the surface of the concrete, result-
ing in shallow cover. is is called improper cover, and it is
the fourth latent construction defect in this discussion.
A ll of these latent construction defects contribute to
a greater or lesser extent to the overall durability of the
concrete structure, but in a synergistic fashion. Concrete
Improper cover is the final defect. This occurs when not enough
concrete protects the steel reinforcement. These hidden defects can
cause concrete to under-perform.
The wrong design mix is another defect. If it's too fluid, layers may
form separating the aggregate and fine materials. Concrete that's too
stiff is difficult to place uniformly.
If the wrong design mix is a problem in the concrete, vibration may
compact the layers incompletely, resulting in "honeycomb" voids.